Many common mass spectrometer products have been developed since the development of mass spectrometry. In an existing mass spectrometer, methods for detecting an ion signal are categorized into: a destructive detection type and a non-destructive detection type. In destructive detection, ions after passing through an analyzer are received by a Faraday cup or a dynode. Charges of the ions are transformed into a current on the Faraday cup, and are amplified by a circuit, or ions are firstly converted to electron and then multiplied by the dynode and their charges are detected. After detection, the ions are neutralized to disappear on the Faraday cup or the dynode. Conventionally, the detection method of this type is used by most mass spectrometers, for example, a quadrupole mass spectrometer, an ion trap mass spectrometer, a magnetic sector mass spectrometer, and a Time of Flight (ToF) mass spectrometer.
When charged particles move to be near a conductor, the so-called “image charges” of an opposite polarity are induced in the conductor, and a current is incurred in a circuit connected to the conductor. By using the method, charges moving near an electrode can be measured, and at the same time of the measurement, the charged particles are not neutralized to disappear. Therefore, the detection method is a non-destructive ion detection method. Recently developed Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometers and Orbitrap mass spectrometers use the method. In analyzers of the two types of mass spectrometers, ions constrained in a magnetic field or an electric field oscillate to and fro, so an image current is induced at one of the electrodes on the analyzer, and a frequency of periodic variation of the image current is a frequency of oscillation of the ions in the magnetic field or the electric field, so that a spectrum acquired by performing the Fourier transform on the image current reflects the mass spectrum of the ions in a trap. Substantially, in the non-destructive detection method, ions can be detected for multiple times in a magnetic field or an electric field within a life cycle of the oscillatory motion, and the time as well as the flight path are effectively increased, so that a very high mass resolution can be acquired.
When reflectors are used in a ToF mass spectrometer, the time and flight path are also effectively increased, thereby a high mass resolution is achieved. Wollnik discloses an analyzer in UK Patent No. GB 2080021A, in which ions fly to and fro between two reflectors for multiple times, and the analyzer is also referred to as a multi-turn ToF analyzer, which has a very high mass resolution. Definitely, the ions are eventually led out to undergo destructive detection after a voltage of one of the reflectors is switched. A problem of the mass spectrometer is that: if a mass range of measured ions is large, the motion cycle time of ions of light mass is obviously shorter than that of ions of heavy mass, and during to and fro movement, the ions of light mass will overtake the ions of heavy mass by one or more turns, so that in the detected mass spectrum, ions of different mass overlap. Therefore, the mass spectrometer can only analyze a small mass range of ions.
By using an electrostatic deflector, a flight tube may also be designed to be of a loop orbit type. In Japanese Patent Nos. H11-135060 and H11-135061, loop-orbit ToF analyzers are introduced. YAMAGUCHI describes a ToF analyzer including a straight out letting flight tube and an 8-shaped loop orbit in US 2006192110 (A1). However, the aforementioned devices also have the problem of small mass range.
Although we can use a mass pre-selection method to limit the mass range of ions to entering the analyzer, and then stitch many mass spectra of a small range into a mass spectrum of a wide mass range by software, many difficulties will be encountered during practical operation, for example, mass errors occur at joints. It is neither easy to introduce an internal mass standard for calibration, and high-precision mass analysis cannot be achieved. In US2005092913 (A1), Ishihara discloses a method of using multiple overlapping mass spectra of difference turns to resolve non-overlapping mass spectra. However, the method requires spectrum acquisition to be performed on a sample for multiple times in different instrument settings, and during the multiple times of the spectrum acquisition, it must be ensured that components of the sample do not change, which obviously brings difficulties to application, and affects the efficiency of analysis.
When a non-destructive detector is used, ions of different mass and ion signals of different turns can be detected by only injecting sample ions once, and a mass spectrum can be acquired by certain conversion methodology. The method has been successfully implemented in FTICR mass spectrometers and Orbitrap mass spectrometers, so is also applicable to a ToF type mass spectrometer. H. Benner discloses an electrostatic ion trap in a U.S. Pat. No. 5,880,466A, which is in fact an electrostatic flight tube having two reflectors. Ions are reflected to and fro between the two reflectors, and the ions have a very high velocity in a drift region between the two reflectors. When the ions pass through a cylindrical electrode, image charges are induced on the electrode, and a circuit connected to the electrode can detect a pulse signal. Zajfman describes in a patent entitled “ION TRAPPING” (WO02103747 (A1)) an electrostatic ion beam trap having two reflectors, and acquiring an image current by using a ring detector. An ion mass spectrum is acquired by performing the Fourier transform on an image current signal.
Intensity of an image current is normally very low. Even if an ion source generates 104 ions of the same mass-to-charge ratio, and the ions move in a compact group, a pulse image current signal thereby generated can just be detected by a low-noise amplifier. However, after multiple times of to and fro movement, the ions in an ion group disperse gradually due to differences in their initial kinetic energy, the image current signal broadens in time and decreases in intensity, until becoming undetectable eventually. The longer the record time of the image current signal is, and the larger the number of times of detection is, the higher the precision of mass spectra acquired by conversion will be. Therefore, it is hoped that ions move to and fro in a flight tube for hundreds or thousands of times. In order to prevent an ion signal from attenuating, Zajfman proposes using nonlinearity of reflectors and coulomb interaction between ions to achieve bunching of an ion group, so as to enable the ions flying in the flight tube not to disperse after hundreds of times of to and fro motion. However, when the bunching based on the coulomb interaction is applied to a mass spectrometer for analyzing a complex ion combination, and especially in the presence of many satellite peaks, large peaks hijack small peaks, which affects resolving power and reduces the precision of the analyzer.
Obviously, in order to improve the sensitivity of the detector, technologies for detecting an image current have to be improved, so as to pick up a sufficient image current signal even when the number of the ions is small.
In addition, effective processing on the ion signal acquired by the detector is also a key to improve the sensitivity of detection. In existing Fourier transform mass spectrometers (for example, an FTICR mass spectrometers and an ORBITRAP mass spectrometer), an image current signal generated by ions of certain mass is close to a sine function or a cosine function, and an image current signal generated by ions of different mass is a superposition of sine wave signals of multiple frequencies, on which a spectrum signal acquired by performing the Fourier transform corresponds to a unique mass spectrum.
When the image current detection is applied for a multi-turn ToF type analyzer, the acquired signal is normally not a sine function or a cosine function. Even a signal generated by ions of a single mass-to-charge ratio has a complex spectrum, which includes a base frequency of the signal and various high harmonics. Therefore, it is necessary to choose a new signal analysis method.